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This invention relates to a frictional vibration damper for use in connection with drill pipes, drill pipe sub assemblies, drill collars, bottom hole assemblies, and drill bits for drilling through subterranean formations. More particularly, this invention describes a vibration damper for downhole tools, particularly drill bits, that will protect cutting inserts and other components from damage due to axial and radial vibrations. These potentially damaging vibrations are produced by drill string flex and high torsional loading and uncontrolled unloading of downhole tools incident to drilling deep oil, gas, and geothermal wells.
Downhole tools such as the drill string which is made of drill pipe, subs, drill collars, bottom hole assemblies, and drill bits, collectively referred to as downhole tools in this application, produce strong vibrations during the drilling process. Excessive vibrations in one tool may adversely affect the performance of other tools as well as the entire drill string.
The dynamics of drilling deep wells are very complex and are affected in large part by the length of the drill string. The length of a drill string used in oil, gas, and geothermal well formation may total as much as 20,000 feet, or more, and is made up of coiled tubing or discrete lengths of drill pipe, drill collars, drill bits, and other downhole tools that are interconnected by tool joints. Each drill pipe is approximately 30 feet in length. Other downhole tools such as drill collars, sub and bottom-hole assemblies, fluid driven hammers, and mud motors vary in length from just a few feet to lengths similar to those of the drill pipe. A drill string weighs according to its length and diameter 100,000 pounds or more and is suspended from the surface by the drill rig. The weight of the drill string is used to drive the downhole tools forward during the drilling operation. This force is generally known as xe2x80x9cweight on bitxe2x80x9d and is controlled at the drill rig by constantly pulling up on the drill string. During drilling, the downhole tools are rotated at a rate of between 60 and 120 RPM, either by a high torque turntable on the floor of the drill rig or by a downhole hydraulic motor, known as a mud motor. Drilling fluid is pumped down the bore of the drill string and exhausted at the drill bit. As the drill bit penetrates the formation, it forms a borehole of a predetermined diameter, or gauge, which is larger than the circumference of the components of the drill string. The area differential between the gauge of the borehole and the circumference of the drill string is known as the annulus, and it provides a path for drilling fluid to return to the surface carrying the cuttings and other debris produced by the drill bit. A suspended drill string is constantly under tension and becomes very flexible behaving much like a spring during the drilling operation. Even though the weight on bit may be several thousand pounds, the drill string may flex longitudinally causing the bit to actually bounce on the bottom of the borehole. The friction along the surface of the drill string caused by the circulating drilling fluid, and the friction from downhole tools rubbing against the wall of borehole, when combined with the natural resistance of the formation to the cutting action of the drill bit, produces a high torsional load in the downhole tools. This load may actually cause the drill string to wind up like a spring. (If weight on bit and torque are not carefully controlled, the torsional load on the drill string will actually xe2x80x9ctwist offxe2x80x9d the drill pipe.) As torque is constantly applied to the down hole tools, the torsional load builds up until the stored energy overcomes the resistance of the downhole tools, and they break loose accelerating to over 300 rpm. For this reason it has been observed that the rate of rotation of the drill string at the surface is not representative of the rate of rotation of the down hole tools at the bottom of the borehole. Because of the flexibility of the drill string, during periods of acceleration, the drill string and downhole tools actually radially and axially bounce, or vibrate, until the stored energy in the drill string dissipates, and the dynamic cycle of loading and unloading begins again. An analogy may be drawn to a capacitor and a resistor in an electronic circuit. The downhole tools, especially the drill bit, store torsional energy that will discharge violently unless controlled by a resistor.
Therefore, unless the stored torsional load in the downhole tools is damped, a violent discharge of energy will occur, manifesting itself as excessive vibrations causes excessive wear and damage to the downhole tools and may also result in borehole deviation. Therefore, it is desirable to develop means for damping vibrations by controlling the torsional loading and unloading in downhole tools.
One means that has been developed is described in U.S. Pat. No. 3,660,990, incorporated herein. This patent teaches the use of a sub assembly having a metallic sleeve slidably housed in splines within the casing. A rubber bush is provided so that when the drill bit receives a longitudinal shock the sleeve moves into the casing assembly and the rubber bush is deformed to absorb the shock. This tool is axisymmetric with the drill string within the borehole. It is not intended to ride on or rub against the walls of the borehole.
Vibrations in downhole tools result from complex forces acting on the tools simultaneously. These vibrations consist of axial, radial, and torsional movements all along the drill string. The preceding patent does not treat in full the complexity of tool vibrations and is limited to damping the longitudinal movement of the drill bit. Also, the mechanism restricts the flow of drilling fluid, requiring higher horsepower uphole and adds a complex, expensive component to the drill string that is prone to failure in the harsh environment downhole due to the invasive nature of abrasive drilling fluids pressurized to 10,000 psi downhole.
Another frictional damper in drilling tools is taught in U.S. Pat. No. 4,428,443, incorporated herein by this reference. Therein is disclosed a shock-absorbing tool having a number of open sections formed in a cylindrical member. The open sections are connected by a plurality of slots. As dynamic axial and torsional loads are applied to the drill string, the tool absorbs these loads through deflection of the tool such that the slots close and the stress is carried about the periphery of the open sections. This tool is deployed axisymmetrically with the drill string and is not intended to rub against the walls of the borehole. In fact, interference with the walls of the borehole would compromise the usefulness of this shock-absorbing tool.
Another frictional damper in drilling tools is taught in U.S. Pat. No. 4,901,806, incorporated herein. In this patent, a sub assembly is taught for positioning in the drill string above the bit. The mechanism provides for rotational torque to be transferred to the drill by a series of helical splines. Dual pistons with corresponding fluid chambers dampen axial forces while the helical splines decouple the torsional forces from axial vibrations to insure proper control of the drill bit. Like the previous frictional dampers, this tool is not intended to rub or ride on the borehole wall.
Another means for controlling bit vibrations is displayed in U.S. Pat. No. 5,755,297, incorporated herein. In this disclosure, lugs or pads are preinstalled in the arms of a roller cone bit as an aid to stabilize the bit. The outside dimensions of the stabilizer pad are substantially equal to, or less than, the desired radius for the borehole. This disclosure teaches that the stabilizer pad may contact the borehole wall in order to stabilize the bit. However, the preferred examples given describe an annulus between the exterior of the stabilizer pad and wall of the borehole in order to avoid sticking; therefore, it is evident from these examples that constant contact with the borehole wall is not desired. Furthermore, this tool fails to address the fundamental cause of bit vibrations which is stored energy in the drill string.
Another means for controlling bit vibrations is found in U.S. Pat. No. 3,915,246, incorporated herein. In this patent a drill bit is disclosed having a drill body with a full gauge, upper stabilizer section and an intermediate taper section tapering to a reduced diameter from the stabilizer section. This disclosure teaches the use of cutting elements having different heights in order to assist the advance of the bit with a minimum of required thrust. It also teaches the use of the upper row of cutting elements to form the gauge of the borehole. A drawback of this design is that as the bit vibrates these aggressive gauge cutting elements tend to cut further into the formation, increasing the diameter of the borehole and contributing to bit instability instead of reducing it. Also as the rotating bit advances, these aggressive cutting elements will form spiral grooves in the sidewall of the borehole producing a thread like condition in the walls that may actually cause the bit to stick during tripping. Moreover, this bit also fails to solve the problem of frequent and rapid bit accelerations resulting from the release of stored energy in the drill string.
What is needed is a means for damping downhole tool vibrations that is economical, robust enough to withstand the native conditions of the borehole, and is not prone to mechanical failure due to the invasive presence of the abrasive drilling fluid. Furthermore, a tool is needed that can use the borehole, itself, to damp the vibrations incident to deep well drilling.
The vibration damper of this invention utilizes the borehole wall as a brake to control the torsional loading and unloading of downhole tools in order damp vibrations and achieve downhole tool stability. In many subterranean formations, the borehole wall may be used as a bearing surface. This invention seeks to use this property of the borehole wall as means to stabilize the downhole tool. The frictional vibration damper of this invention is intended to be used either on the downhole tool, itself, or in connection with other downhole tools. The vibration damper is adapted to receive wear resistant damper inserts that describe a circumference larger than the desired gauge of the borehole. These damper inserts are positioned on the downhole tools in such a fashion that they are constantly forced against the sidewalls of the borehole. They act like a frictional brake that maintains a constant torsional load on the downhole tools and controls torsional unloading in order to prevent violent accelerations of the downhole tools. The inserts may plastically deform the borehole wall by an indenting, brinelling, or crushing action as a means of damping radial and longitudinal vibrations incident to subterranean drilling. The damper may be tapered or the damper inserts may describe a progressively increasing circumference in order to maintain a constant force against the borehole wall during drilling. The working surface of the damping inserts is coated with a superhard material such as polycrystalline diamond or cubic boron nitride and is configured to have a rounded, non-aggressive working surface. These superhard materials assist in controlling the amount of friction and heat produced by the damper. This superhard coating is abrasion resistant and capable of withstanding the heat and pressure generated by the damping inserts. The damper may also consist of a means for cooling the damping inserts using the drilling fluid.